Influence of Moisture, Temperature and
Microbial Activity in Biomass Sustainable
Storage. Special Focus on Olive Biomasses
Pedro J Lara Chaves1, Julio Terrados Cepeda2*, Francisco J Gallego Álvarez2 and Manuel J Hermoso Orzáez2
1 Bioland Energy S.L, Spain
2 Department of Engineering Projects, University of Jaen, Spain
Submission: July 31, 2020; Published: August 11, 2020
*Corresponding author: Julio Terrados Cepeda, Universidad de Jaen / Campus Las Lagunillas, Edificio A3 / 23071 Jaen, Spain
How to cite this article: Pedro J L C, Julio T C, Francisco J G Á, Manuel J H O. Influence of Moisture, Temperature and Microbial Activity in Biomass
Sustainable Storage. Special Focus on Olive Biomasses. Int J Environ Sci Nat Res. 2020; 25(3): 556165. DOI : 10.19080/IJESNR.2020.25.556165
Biomass is a renewable energy source that, due to its high seasonality, needs to be stored, handled and managed in suitable conditions for its optimum use. Sustainable storage is, therefore, a key process in which biomass can lose much of its qualities as fuel. The article presents an exhaustive bibliographic review of the factors that affect the quality of biomass during storage and the interactions that occur among them. Humidity, type of product, granulometry, size of the stockpile, airflow, temperature, and microbial action are analysed as the main factors affecting biomass during storage, and the results are compared with the tests that have been carried out on biomass from olive groves and olive oil industry. Recommendations are presented so that, using a correct storage method, losses, degradation and self-ignition risks are reduced and the energy quality of the fuel could be improved, taking advantage of the storage process to optimize the net energy yield.
Since prehistoric times, humans have used wood and other organic waste as a source of energy. This biomass has been used as fuel, with the name of biofuel, or in general as bioenergy. Hence, biofuels may include a large group of different fuels, which all are originated from biological resources. These biofuels include, among others: wood, bark, chips, sawdust, pulp-mills, straw, waste and peat.
However, the use of this biomass as fuel is not always easy, since biomass is a very heterogeneous renewable energy source that can have different structures and house different compositions of matter. This makes its handling complicated and must be addressed in different ways depending on the type of biomass being used, the conditions in which it is worked such as climate, the spaces and surfaces, and also on the techniques used to store the product, conserve it, dry it and mechanize it. All these conditions will eventually influence the energy quality of the biomass.
Most of the studies carried out on the energy use of biomass agree that the storage stage is a fundamental phase given that during it there are alterations in the quality of the product and
loss of dry matter, which will be greater the worse the storage is carried out .
Therefore, the use of a good storage method will greatly reduce losses , reducing in turn the economic losses associated with energy use of biomass, as noted in the study by Ronald Gonzalez . On the other hand, other studies show that the storage of biomass in the field can be used to improve fuel quality. For example, the study by Shuva Gautam  shows that the quality of wood changes during storage and also its net energy yield. They assessed the main fuel attributes (moisture content, gross calorific value and ash content) during the storage of logging residues, and showed that the storage regime played a significant role on the fuel quality of such residue, and that proper storage may improve the fuel quality and net energy yield, leading to an overall cost reduction of the biomass feedstock.
Thus, the objective of this study is to make a bibliographic review of those factors that may affect the quality of biomass during storage, analysing the interactions that occur between them, and establishing their degree of importance. Likewise, the results of this review will be compared with those obtained from field tests carried out with olive biomass in an electricity
generation plant in the south of Spain.
The storage of biomass in its various configurations (chips,
pellets, ...) has been investigated at various levels over the last few
years. These researches have brought to light the existence of a
wide range of factors that may affect the quality of biomass during
Good quality biomass must be homogeneous, low in moisture
content, low in ash content, and with little foliage or particles,
and these factors could be altered by storage conditions such as:
shape, location, duration and weather conditions .
Most of the authors [4,6-10] agree that the main factors that
determine the loss of quality of biomass during storage are the
following: type of material (hardwood, softwood, leaf and/or bark
fractions), size of the stockpile, initial humidity, degree of heating
and self-ignition risk, particle size and distribution, stockpile
ventilation, climatology (ambient temperature and humidity),
wind speed and direction, and the action of microorganisms
(microflora, insects and bacteria). All these factors interact with
the chemical composition of the biomass and most are related to
Furthermore, Gold and Seuring analysed the most relevant
issues of supply chain management and logistics for bio-energy
production  and among their findings they assure that the
main risks to be minimized in a storage system are biomass
quality degradation and dry matter losses.
The study developed by Emery & Mosier  focus on dry
matter losses and show that the main losses are associated with
moisture content, exposure to rain and contact of the pile with the
soil. They assert that indoor dry storage generates the smallest
losses, while outdoor storage allows greater losses up to 16%,
with the potential for extremely high losses of over 36%. On the
other hand, the Gold and Seuring  investigations broaden
this range of factors, proposing that there are others that affect
the cost loss of the product during storage, such as location, type
of storage and the number of manipulations carried out on the
The location of the storage can be placed in areas close to
the harvest or collection field, on the road or some more specific
centres such as port silos. While the type of storage can vary from
open-air storage to closed-roof storage with fans.
The choice of storage type will depend on the environmental
climate and the biomass processing stage. Studies by Thornley
 and Rentizelas et al.  highlight that for humid climates
and Mediterranean climates, respectively, covered facilities can be
assumed, usually for short rotation storage.
Other factors affecting storage will be the volume and density
of biomass to be stored , the duration of storage phase ,
and the proportion of wood, bark or foliage [16,17].
According to existing research on biomass storage, and as
we will see in the following sections, it can be seen that the main
factors affecting the quality of biomass during storage are the
initial moisture, granulometry, herbaceous component and the
size of the stockpile. Moreover, within the factors that appear by
causality of these, we must highlight the temperature as the main
factor to be taken into account, as it is the cause of self-ignition
Most of the existing studies reveal that the physical-chemical
factors are linked to each other and that the variation in the
parameters of one of them causes changes in the rest. According
to M Pettersson  the most important characteristic of biomass
is its moisture content, since it affects the calorific value, storage
properties and transport costs that finally determine the price of
Biomasses with high moisture content have a higher activity
inside the pile during storage. If no heat exchange is generated
between the interior and the exterior during storage, the increase
in temperature increases the risk of internal reactions and can
cause self-ignition .
The humidity of the air also influences the moisture content of
the biomass. Changes in humidity are governed by the equilibrium
relationship between wood and air. Compared to humidity, air
temperature is not such a significant factor. If it can be when it
comes to storage for short periods, however, other factors are
more important when it comes to storage for longer periods, as
shown by the studies of Lin & Pan 
There are also other factors associated with biomass moisture
that can favour degradation and self-ignition processes during
storage. In the Yasuhara study  it was concluded, among
others, that not only the water content of the biomass was the
cause of self-ignition, but also the initial temperature and particle
size are indicative factors.
Most authors therefore agree that the moisture content of the
biomass is the main factor that generates temperature increases
within the stockpile. Moreover, this increase in temperature is a
determining factor in internal reactions and risk of self-ignition.
Humidity is also considered by Hakkila  as the most
influential factor. In his analysis, based on the energy and climate
strategies implemented in Finland, he ensures that moisture is the
most important factor in the quality of wood biomass because it
a) Price: the higher humidity is, the lower is biomass price.
b) Calorific Power: higher humidity implies lower calorific
c) Crushing and/or handling: depending on the type of
machinery, it will be easier to grind or handle a dry or wet product.
d) Transport: less humidity, less weight transported in the
e) Storage: higher humidity, greater degradation of biomass
properties throughout the storage period.
f) Energy production: loss of biomass properties directly
affects plant energy production.
g) Profitability: negative effects on production are
transferred to profitability.
h) Emissions: higher humidity implies greater possibilities
of incomplete combustion and carbon monoxide emissions.
Other studies, such as the one conducted by Towey et al.
, also conclude that biomass with moisture content below
20% are stable and not fit for microbial consumption, regardless
of environmental temperature, while biomass with moisture
contents above 20% may degrade and would not be put into longterm
In addition, the moisture content increases with decreasing
particle size and increases with time of contact with air. Therefore,
granulometry, height and stack size are very important factors in
this process [6,23].
Another determining factor is the storage duration. Various
studies analyse moisture variation over time [8,24]. In relation
to this variation in humidity, the storage of biomass small piles
during medium-long periods (up to 9 months) may recover up to
4% of energy through loss of humidity. In large piles the opposite
occurs, energy is reduced by 3% and in a shorter period (up to
6 months). The greatest losses occur in large stockpiles and
in those where the particle size is small. The ideal height for
non-compacted woodpiles would be 6 to 16 meters, while for
compacted piles it would be 4 to 12 meters.
Air temperature is another major factor in the biomass drying
process. Several authors have studied the relationship between
environmental factors and the humidity of the stored biomass.
Pari et al.  analysed the relationship between wood moisture
content, air temperature, air humidity and rainfall, concluding
that air temperature affects wood moisture content by 70%, while
air humidity and rainfall are barely significant. Generally, weight
loss is highly temperature related. The weight loss ratio is 1 - 2%
per month .
Zabetti et al.  also highlights how the ISO classification
maintains as biomass quality parameters those strictly related to
the energy content of the wood chips, such as moisture content,
while it is more tolerant in terms of particle size classification.
As it has been commented, although it is a factor caused by
the combination of others, it is worth to analyse the temperature
and its relationship with the other factors. According to Ferrero et
al. , who developed a model for predicting the heating-up and
the possible self-ignition of large-scale wood piles, the increase
in temperature inside biomass stockpiles can be generated by
three different causes: by oxidation reactions inside the pile, by
exothermic condensation/adsorption processes originated inside
the stockpile, or by microbial processes.
According to a study carried out by Thörnqvist , the
temperature in small piles, up to 120m³, is kept approximately
at ambient temperature, while in larger piles, up to 600m³, the
temperature is usually between 10 and 30ºC above ambient
temperature in the centre of the pile. However, this behaviour
may be influenced by external conditions. In a study conducted in
Canada  in which birch biomass was stored in different forms,
it was observed that, due to good air flow conditions, there was no
increase in temperature in the wood piles, following the pattern of
Is worth mentioning that the number of woody biomass
related accidents are mainly related with temperature increase
and self-heating. Krigstin et al.  conducted a compilation of
incident reports involving biomass storage, and such study has
revealed that these potential hazards continue to be a major
concern in the bioenergy sector. They also state the need of realtime
pile monitoring in order to provide instant temperature data
on to properly control such incidents.
In the case of the self-heating of large scale storage of
biomass, it has been reported the influence of chemical oxidation,
microbiological processes as well as the effect of the evaporation
and condensation of moisture as mayor factor increasing risk of
ignition . In such cases, moisture content has to be controlled
and inside temperature needs to be monitored as there is evidence
that periodic behaviour occurs in large stockpiles because of the
interaction of the evaporation and condensation of water together
with chemical reaction.
Other factors, such as the content of bark or leaves that means
higher herbaceous component, may also increase the temperature
during storage. In the study carried out by Anheller  with data
from the storage of pine bark in both small and large stockpiles,
the stockpiles suffered a high and rapid temperature increase,
reaching 60°C in a few days in small stockpiles and above
70°C-75°C in large stockpiles.
Moreover, the temperature distribution is usually not
homogeneous within the stockpile. Buggeln  showed
temperature distribution in stockpiles 4.5-6 meters high. The top
and centre of the stockpile reached temperatures above 60°C,
while the bottom of the stockpile never exceeded 50°C. Thus, heat
loss occurs in the upper and lateral parts of the stockpile.
On the other hand, air circulation and storage duration are
factors described by Gislerud & Grønlien  as determining
factors in the temperature increase inside the stockpile. In the
analysis carried out on a wood chips stockpile of about 500m³
it was observed that the temperature increases over time in
all the measurements and, depending on the area where the
measurement is made, the temperature increases at some points
much more than at others as a function of the circulation of the
These factors have also been proven in other studies. Hogland
& Marques  studied the temperature increase of wood piles to
the point of self-ignition and analysed the temperature variation
at different points from the surface to the inside of the stockpile.
The highest temperature was measured in the internal-high parts
of the collection, passing in the first month from 49°C to 73°C.
Then it increased very slowly and progressively from 66°C to
90°C on average. In this way, it was maintained for 5-6 months
until a strong wind propitiated a self-combustion, increasing
the temperature drastically up to 246°C in 24 hours. These high
temperatures over long periods of time propitiate self-ignition of
the biomass. Yasuhara et al.  found that biomass can burn on
its own if subjected to temperatures above 140.5°C and exposed
to it for 270 minutes. This drastic increase in temperature was
also experienced in a stockpile of 5,000m³ analysed in the study
carried out by Löwegren  in which the temperature increased
in several days to almost 300°C, when it had been stored for about
There is no exact self-ignition temperature of the biomass. It
depends on the type of material, catalyst action and oxygen input.
One of the main reasons for self-ignition is the permeability of the
product, since it can retain water in the interior that makes heat
and temperature transfer to other parts . The first signs that the
stockpile is in poor condition and that there may be self-ignition
risk is the presence of strong odours and high temperatures.
The Frank-Kamenetskii method, reviewed by Per Blomqvist
and Bror Persson in detail , assumes that self-ignition process
occurs if the material is sufficiently porous and reactive to
allow availability of fuel and oxygen throughout the whole selfheating
process. It defines a characteristic and critical length of
the pile above which the material will self-ignite as a function of
the ambient temperature, but no account is taken to moisture
transport and the accompanying phenomena of hydrolysis,
evaporation and condensation. Blomqvist and Persson assert that
the Frank-Kamenetskii theory is best suited for application on
materials that do not have excessive moisture content and where
the reactions can be sufficiently well described by a single order
In general, there is a great increase in temperature in the
internal and superior areas of the pile. This is due to the fact that
with the passage of time both the humidity of the biomass and the
microbial activity increase the temperature inside the collection.
If the air circulating from the outside of the stockpile to the inside
does not reach the internal areas, it does not provide the necessary
cooling and self-heating may take place. In addition, if the air
ambient temperature is high, it contributes to the acceleration of
the temperature increase. In woodpiles, temperature and humidity
are transported by the flow of air to the more internal-high parts of
the collection, increasing the temperature by conduction and the
humidity by condensation of gases and vapours . In the central
parts of the stack, the temperature increases rapidly during the
first weeks, then stabilizes and finally decreases progressively .
Due to the circulation of air through the pile during the drying
process, a “chimney effect” is produced in the upper areas. The
air enters the pile cooling the product, but reheating as it moves
through the collection. The air carries away the water contained
in the materials, thus creating a flow of moist and warm air. This
makes the water flow vertically up to the top of the collection
where it undergoes a strong heating and is expelled to the
outside in the form of steam. This reheating is favoured by the
microorganisms dragged by the airflow and, if the air circulation
is vigorous, there may be penetration into the particles and a heat
transfer effect may occur [9,37]. Heat can have a positive effect
if the dissipation becomes homogeneous, as it can decrease the
humidity to all parts of the pile and increase its energy power, but
with the “chimney effect” the inner parts dry out while outer and
higher parts wet out .
These effects can be seen in Figure 1, which was taken from
the tests carried out by the University of Jaen, where the action of
the airflow through a biomass stockpile is observed.
The height and compactness of the piles also has a great
effect on temperature variation. The higher the pile height, the
greater the compaction and therefore the ventilation is lower, the
temperature increases, and spontaneous combustion may occur.
On the other hand, smaller piles are poorly compacted and their
temperature does not usually exceed 54ºC, although this favours
The increase in the compaction degree of the biomass pile
prevents air circulation and has two opposing effects : the
lack of heat evacuation by convection, which produces heating,
and the decrease in biological activity, which produces a decrease
Hence, compaction influences the permeability of the pile
against an airflow. This permeability, according to Ernstson ,
depends on both the porosity of the material and the compaction
of the pile. On the other hand, the interaction between the
environment and the stockpile will be greatly influenced by the
compaction of the pile. Gautam  compares hardwoods and
softwoods, where hardwood stockpiles have a larger empty
space than softwood stockpiles. The more pronounced branching
of hardwoods allows for less compaction, therefore, there is a
greater percentage of free space which implies less resistance to
air passing into the stockpile. In softwoods, the voids are smaller
and therefore there is greater resistance to airflow, especially
when the stocking form is not in a row. It is also noted that the
shape of the stockpile has a significant effect on the moisture
content of the stockpile. Such study shows that stockpiles in rows
have a larger area exposed to ambient air, which favours drying,
cooling and moisture loss.
In short, the granulometry of the material affects the
circulation of air through the collection and conditions the
increase in temperature. According to the study carried out by
Ferrero , in which the factors that intervene in self-ignition
during the storage of chipped wood stockpiles are analysed, it is
observed that the temperature increases to a greater extent and
is maintained over time, reaching over 70°C, in stockpiles with
finer granulometry. While, in larger granulometry stockpiles,
temperatures do not exceed 45°C and after less than one month, it
begins to fall and equalize to the ambient temperature.
This temperature increase has also been analysed by Fuller
 in a study where he obtains a sequence of temperature
increase in biomass stockpiles. It is based on the fact that the main
factors that produce this temperature increase are the compaction
and the height of the pile. It is also noted that the living cells are
in the bark, foliage and wood. Therefore, when the tree is crushed
these cells (Parenchyma) breathe in an attempt to heal the tree
and can live up to six months depending on conditions. In this
process, oxygen is consumed, and heat is released, generating
good conditions for the growth of bacteria. For this reason, several
authors agree that, for crushed products, the size and height of the
pile must be smaller in order to reduce the risks of self-ignition
Other authors also focus on the effects of particle size on
ventilation and self-ignition risks. Ramirez et al.  considers
that the humidity of the product, the size of the particles and
the place where the product is stored significantly affects its
flammability and there are significant differences in storage
caused by the difference in particle size. Furthermore, the greater
porosity of the collection composed of larger particles improves
the natural aeration of the storage .
The effect of granulometry was also analysed in a study
developed by Anheller  where two biomass storages of
different sizes were carried out. One of the piles was made of
recovered wood (6.5m. height x 50m. length) and another was
made of bark (4m. height x 15m. length). It was observed that in
the first pile there were no significant changes in temperature and
humidity. While in the collection of bark there was an increase in
temperature in the first days, reaching up to 75°C. In the bark piles
there are more losses due to moisture content and particle size.
Temperature is not the only indicator of deterioration and loss
of dry matter. It is also necessary to take into account the presence
of fungi, bacteria and microorganisms. Therefore, the chemical
composition of wood is very important, especially if it is to be
stored for long periods of time .
Different types of bacteria can be found in the same pile of
wood, since, within the same pile, the temperature also varies
according to whether the zone is upper, lower or medium
. Depending on the temperature of the different zones of
the collection, a series of microorganisms or others will act:
Cryophylls, between 0°C and 20°C; Mesophylls, between 20°C and
30°C; and Thermophiles, between 35°C and 70°C.
The composition of the biomass also influences the presence
of microorganisms. The presence of lignin hinders the action of
bacteria on cellulose and hemicellulose to complete pentose.
Delignification is more difficult in soft woods and access to
cellulose chains is a problem. Hardwoods have higher acetyl
(acetic acid) and higher levels of pentose, which favours bacterial
activity. In softwoods, the low level of pentose can prevent rapid
initial pH changes, although high lignification can reduce the
efficiency at which hemicellulose could produce xylenes.
The study conducted by Gautam  shows that softwoods
produce less moisture content than hardwoods, due to their
chemical and anatomical composition. In hardwoods hemicellulose
constitutes 25%-40% compared to 25%-30% in softwoods.
Hemicellulose is the most hygroscopic component of cell walls,
followed by cellulose and lignin. Because of this, hardwood cell
walls will have more potential accumulation sites for water, which will favour moisture increase and bacterium action.
Organic matter is broken down or decomposed by bacteria
or microorganisms to CO₂, H₂O and energy. This released energy
is used by the cells for their metabolic process . The much
more common storage of chips in the open-air leads to greater
importance of chip size and the proportion of fine particles. For the
finer the chips, the more the air movement will be altered and the
less heat dissipation from the stockpiling will occur. Because of the
heat produced in the stack, there will be more fermentation and
more pentose will be produced from cellulose and hemicellulose,
which would produce lactic acid, acetic acid, water and carbon
dioxide. However, if the particles are larger, there is more aeration
inside the collection. This aeration also makes more oxygen
available within the collection, which is an important requirement
for the biological and chemical decomposition process .
When wood is cut or crushed, living cells tend to repair the
damage by increasing respiration and thus increasing the heat
released. In compacted stockpiles there is much more wood,
which means more heat to dissipate. This heat generates good
conditions for the development of bacteria. The cutting and
fragmentation of wood facilitates the process of decomposition
and degradation, as they have more surface to be attacked by
microorganisms. Between two and five centimetres, it is the ideal
size. The presence of thick branches or lignified elements can
inhibit, slow down and delay the fermentation process .
The compaction of the collection will depend largely on the
granulometry of the material. Greater granulometry means
greater contact surface for the proliferation of fungi, although on
the other hand the effect of compacting decreases the passage of
air . In ventilated piles, the risk of self-ignition and moisture
content are reduced, but a favourable environment is created for
the establishment and growth of different species of fungi despite
the low humidity .
A parameter that evidences the changes that are taking place
in the biomass by effect of the fungi and microorganisms is the
colour that such biomass presents. Eslyn  studied the colour
variation of biomass depending on how fermentation progressed,
and differentiated between:
a) Light brown colour - medium: when there had only
possibly been an oxidation of the surface of the biomass.
b) Dark brown colour: when a soft rot begins to occur.
c) Blue spots: there is more rotting and more action of
bacteria, fungi and microorganisms.
d) Bleached chips: there is more rotting which also affects
the hard parts of the wood.
In the tests carried out at the University of Jaen it has been
possible to verify this evolution of the colour in the superficial
part of the biomass stockpiles which allows us to know whether
or not there is bacterial or microbiological action:
a) Bronzed colour: there is hardly any biological action
b) Color White: action or appearance of fungi (Figure 3).
c) Colour Black/Brown/Yellow: action of different
microorganisms (Figure 4 & 5).
Thermogenic processes occur during storage, mainly due
to the action of living wood cells, the biological action of wood
and the phenomena of chemical oxidation and acid hydrolysis of
cellulose components .
Normally, the initial temperature increase in a biomass
stockpile occurs because of biological action. Bacteria, in the
right environment, produce fuel degradation by generating heat,
carbon dioxide and water. They can act up to 60°C but at higher
temperatures, their activity decreases rapidly. The most important
are the fungi, and among them, the fungi that usually act on the
biomass are the xylophage’s fungi and blue fungi. The former
produce greater degradation and at a faster rate. They normally
live in temperatures between 20°C and 40°C but they can survive
at temperatures of -2°C and above 50°C. Furthermore, fungi have
to work in the presence of water.
On the other hand, bacteria tolerate a wide range of
temperatures, reaching up to 75°C [6,8]. When the temperature
exceeds 40°C chemical reactions begin, and at 50°C the most
important ones begin to take place. Biological activity increases
the temperature to levels at which chemical action takes part .
In wood storage, if the wind hits a stockpile, or part of it, that is
at temperatures close to 80°C, oxidation processes accelerate and
the temperature may increase very rapidly .
If biomass is stored uncrushed, little heat develops. In this case,
there is no friction between particles, the specific area of action of
the microorganisms is smaller and the cells of “Parenchyma” have
very little oxygen for their respiration .
According to studies conducted by Emery et al. 
fermentation, which reduces the pH of the pile, occurs during the
first week of the first month of storage. After this, the collection
will be relatively stable for up to one year with minimal losses if
good storage practices are followed. Therefore, losses are believed
to be resistant to variations in storage duration
In addition, microbial activity slows down in acidic media
with pH less than 6. Therefore, it is advisable to maintain low
pH levels so that there is no microbiological action. To prevent
microbial action, fungicides can be added but this could lead to
environmental and legislative problems . Another solution
to mitigate biological activity is to keep humidity below the
saturation point of the fibre, which is 23% . It is also possible to
increase the temperature up to 70°C or more, but in this case the
quality of the wood may decrease.
Decomposition effects are more likely to occur when the
product contains leaves, bark or foliage [50,51]. Wood chips have
a long surface area that allows bacteria to colonize. Forest residues and barks contain high concentrations of minerals and inorganic
elements that could also improve microbial activity. Stockpiles of
products with high bark or herbaceous content begin to undergo
chemical and biochemical reactions before three or four weeks of
The Emery and Mosier study  assumes that dry matter
loss is primarily the result of biological degradation and
suggests that renewed microbial activity can cause losses of
1.5% of the total when more than 48 hours elapse from product
storage to consumption. Normally, the balance between quality
improvements due to biomass drying and dry matter losses due
to bacterial action is usually negative. They will depend on the
humidity and the storage time duration .
Other studies  also contrast that the main losses of dry
matter are caused by bacterial action and the loss of product during
handling and transport. In some cases  wood losses have been
estimated in the range of 0.5%-1% in cold and temperate climates,
while they would be 0.75%-3% in hot and humid climates.
At higher temperatures, between 35°C and 50°C, the
concentration of gas, generated inside the stockpile and emitted
to the outside, increases significantly. At 35°C, the concentration
of CO and CH4 are always high, while that of CO₂ is only high in
some cases. This is due to microbial activity . Therefore, the
storage temperature is a crucial factor during the storage of fresh
waste. High temperatures lead to high concentrations of gases and
the concentration of gas has a high relationship with the loss of
dry matter from the stored product .
The University of Jaén, in collaboration with the company
Valoriza, has carried out a series of tests with different types of
biomass from the olive grove and olive oil sector, and it has been
possible to verify in them the parameters analysed previously.
It should be highlighted that more than nine million hectares
of olive trees are cultivated throughout the world, especially in
Mediterranean countries. As an essential operation, the pruning
of olive trees produces an enormous amount of biomass that,
in most of the cases, lacks industrial applications and must be
eliminated annually from the fields to prevent the spread of plant
diseases. The biomass produced by pruning is usually eliminated
by direct combustion or by chopping and dispersal in the field
The importance of the biomass potential of olive groves in
the south of Spain, especially in the province of Jaen, is enormous.
Several studies have quantified the production of biomass as a
by-product of olive cultivation in a range of between one and five
tonnes per hectare, which means an annual production of about
800,000 tonnes per year . This is the main justification for the
research work that is being developed at the University of Jaen in
order to optimize and valorise energy production based on the
use of these biomasses.
Tests were carried out at Valoriza Energía’s facilities, located
in the province of Jaen (Spain). This biomass power plant has
more than four useful hectares of biomass storage and consumes
nearly 120,000 tons of different types of biomass per year. With a
nominal power of 16MW, combustion takes place on a vibrating
grate cooled by water and by the combustion air itself. Although
this plant is designed so that its biomass consumption can be
composed by 100% of olive pruning, in this plant poly-biofuel
mixtures are also used.
The general characteristics of olive grove biomass are shown
in Table 1.
The tests on biomass storage processes were carried out
between November 2015 and April 2017. We worked with 33
stockpiles of biomass that were stored between 20 and 452 days,
before being processed in an electricity generation plant. The
main characteristics of such stockpiles are accounted in Table 2,
including storage input date, total storage time, initial humidity,
mean temperature, weight and higher heating value.
Tools used for data collection were the following:
a) Initial Humidity (%). Moisture analysis was carried out
according to UNE-CENT/TS 14774-1/2/3, by drying the sample in
oven at 105ºC up to constant weight.
b) Initial Higher Heating Value (HHV) (kcal/kg). The
HHV was analysed by means of a PAR 6000 Calorimetric Pump
according to Standard UNE-EN-14918.
c) Initial Lower Heating Value (LHV) (kcal/kg). According
to the HHV data obtained in the calorimetric pump and according
to the UNE-EN-14918 Standard.
d) Temperature control by means of a FLIR ix series Extech
IRC30 Thermographic Camera.
e) Temperature evolution inside the collection. By means
of a temperature probe placed in the stockpile that informed of
what was happening inside the collection.
The crushed pruning of olive tree is a by-product with
high herbaceous content and small granulometry. These
characteristics favours self-heating and fermentation. The storage
of this kind of biomass must be carefully controlled since its state
and progression will depend much on the initial conditions of
humidity and size.
After carrying out the tests and trials with this product, the
following observations were obtained:
a) Biomasses with initial humidity above 20% generate
temperature increases above 80°C. While a lower initial humidity
allows the storage of larger stockpiles, without exceeding 1,000 -
b) After two months of storage, the rate of moisture loss is
c) The larger the piles are the higher temperature is
reached inside. Thus, in stockpiles larger than 800 Tons, the rate of
moisture loss is reduced and the inner temperature increases. In
large stockpiles, as the temperature increases there is degradation
of the biomass, which is checked with the decrease of the higher
d) Small and high moisture piles lose a lot of weight in a
short time due to drying and moisture losses. This weight loss is
not proportional to the calorific power gain so the energy balance
may be negative.
e) Small granulometry implies less moisture loss and less
cooling of the collection.
f) The herbaceous composition of the product increases
the action of bacteria, i.e. fermentation.
This biomass, which has a larger granulometry than
the previous one, must be crushed for final use just before
consumption or final use. The global mean temperature is ten
degrees lower than the one of crushed pruning biomass.
A number of observations have been made of the tests carried
out with this type of product:
a) Stockpiles with humidity values below 15% hardly lose
any additional humidity, while for piles with humidity values
above 20-25% the loss of humidity during storage is quite large.
In large stockpiles, the rate of moisture loss is reduced, while in
stockpiles of between 800 and 1,000 tons the rate of moisture loss
is higher the higher the initial humidity is. The rate of moisture
loss decreases as the storage duration increases.
b) In large piles, the temperature increase to almost 80°C,
while in smaller piles the temperature does not exceed 60°C.
c) In smaller stockpiles, the degradation is much less than
in larger ones, although in general the degradation that occurs in
this product with this granulometry is very low, unless it is stored
for a long time or is very wet
d) If stored for more than one month after being crushed,
problems of fermentation and self-heating may occur if the
humidity is not completely low (Parechyma).
e) The entry of air into the collection is much easier due
to its higher granulometry, so the aeration is much higher. This
allows the accomplishment of bigger stockpiles.
f) It is very difficult for self-heating to occur, but when
it does, it is very dangerous as it has a large air inlet into the
stockpile, which intensifies the spontaneous combustion process.
The use and storage of biomass for energy generation is a
sustainable practice, as long as appropriate methods and controls
are used. The action to be taken on biomass during the storage
process, from start to finish, will depend mainly on the physicalchemical
characteristics that the product is collected with.
Most of the studies agree that biomass moisture content is the
most important characteristic as a crucial factor that generates
temperature increases within the stockpile, putting at risk the
quality of the biomass during storage.
Depending on the moisture content of the stored biomass, the
effect generated on the stockpiles can have different behaviours.
If the initial humidity is high, more heat is generated inside
the pile and, therefore, the temperature increases. Moreover,
this increase in temperature is a determining factor in internal
reactions and risk of self-ignition. It also increases, with humidity,
the degradation produced by the action of bacteria, fungi and
microorganisms, in addition to that produced by chemical
reactions at high temperature.
Stockpile size is another critical factor. Wet biomasses should
not be stored in large piles because, in this case, the airflow is
lower and the dissipation of heat generated inside decreases
and favours the action of microorganisms and the self-heating.
On the other hand, a cold environment and good air circulation
may prevent the accumulation of heat and may help to minimize
bacterial activity and self-ignition risks.
With respect to granulometry, the presence of small
biomass particles favours an increase in humidity compared to
environmental conditions. While larger particles favour drying
and reduction of moisture content. Different behaviour of crushed
pruning and pre-crushed pruning of olive tree in storage tests
clearly shows the granulometry influence.
Biomass is easily degradable, especially such kind of biomass
whose herbaceous component is greater. The combination of high
herbaceous content and high moisture content makes the action of
bacteria, fungi and microorganisms increase greatly, significantly
affecting the quality of the biomass.
Regarding microbial action, it is necessary to take into account
other factors: hardwoods are easier to decompose by bacteria and
generate more moisture during decomposition; leaves, foliage and
bark have a high content of nutrients and minerals that promote
microbial activity; the activity of fungi is greater in stockpiles of
chips and this activity contributes to the increase in temperature.
In addition, microbial activity in biomasses with large particles is
low, but when initiated it spreads rapidly.
Hence, optimal size and type of storage is determined primarily
by the combination of moisture and biomass granulometry. For
biomasses with small granulometry, high herbaceous content
and high humidity, small stockpiles are recommended because
in these cases, small granulometry generates more compaction,
less permeability and less dissipation of the heat released. On the
other hand, the pile size affects self-heating as it reduces airflow
in larger piles, while the use of biomasses with large particles
reduces the risk of self-heating.
The tests that have been carried out with olive tree biomass,
in different configurations of stockpiles and durations, confirm
these results. In the case of crushed pruning of olive tree, which is
a biomass with a high herbaceous content and small granulometry,
the temperature increases are very dependent on the initial
humidity. When it exceeds 20% there have been very significant
increases in temperature, exceeding 80ºC inside the pile, which
has favoured self-heating and fermentation. The global mean
temperature is ten degrees higher than the one of pre-crushed
In the case of pre-crushed pruning of olive tree, which is a
biomass with greater granulometry, temperature increases, and
degradation are lower. In large stockpiles, the temperatures have
not exceeded 80°C, while in smaller stockpiles the temperature
does not exceed 60°C. In addition, the degradation that occurs in
this product with this granulometry is generally quite low unless
it is stored for a long time or is very wet. As the inlet of airflow into
the stockpile is easier due to its higher granulometry, aeration is
higher and it allows the formation of larger stockpiles and reduces
the risk of self-ignition.
All authors contributed to the study conception and design.
Material preparation, data collection and analysis were performed
by Lara Chaves, P.J. and Terrados Cepeda, J. The first draft of the
manuscript was written by Lara Chaves, P.J and it was refined by
Terrados Cepeda, J. Hermoso Orzáez, M.J. and Gallego Álvarez F.J.
All authors commented and discussed on previous versions of the
manuscript. All authors read and approved the final manuscript.
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